Estimates of concrete strength are often made from compression tests on cores which have diameter considerably less than the recommended 100 mm. This paper examines the results of tests applied on the 75 and 50 mm diameter cores. A total number of 300 cores were tested. The effects of both specimen sizes and strength levels for different length-todiameter (l/d) ratios of 2, 1.6, 1.4 and 1 on the compressive strength of smaller diameter cores were analyzed. The test results obtained from core specimen were compared with those of standard specimens. The test results showed that the core size had negligible effect on the strength of the cores. The age of the concrete was found to be an effective factor inthe interpretation of the core strength results. Test results also revealed that the l/d ratio of the specimen is more effective for small diameter cores. The coefficient of variation of strength values was noticed to be somewhat higher for 50 mm diameter cores.